Under a polygon's properties, there are three parameters for sound (Sound Source Index; Ambient Sound Index; Random Sound Index). By default they are set to 0;-1;-1. I opened Waterloo Waterpark and copied the way things were set up for sound on the opening polygon (where you start). Testing in the game my Obed map has no ambient sound, while in the standard M2 map everything's ok.

I've also noticed that if i save the map and then open the properties again, the sound indexes are reset to default. So maybe it's Obed debugging them upon save (though nothing shows up in the log to suggest that).

I should also mention that the placed Sound Objects do not work either; however normal weapon, monster, platform, and liquid sounds do work.